In recent years, there is progress in speeding up the processing speed during operation in addition to high integration of CPU or LED semiconductor devices and there is a tendency whereby the amount of heat generated during operation of these devices increases compared to conventional devices. As a result, if heat which is generated during operation of a semiconductor device is not efficiently released to the outside, there is a risk that the internal temperature of the semiconductor device during operation may exceed the limits of the operating temperature. Therefore, cooling by interposing a heat transfer composite material having a thermally conductive filler dispersed in a matrix between heat generating components and heat dissipation components is generally performed.
In the case where the particle shape of a thermally conductive filler is not spherical but fibrous, the surface area per unit weight increases. As a result, in the case where it is filled into a resin material, it is easy for thermally conductive filler particles to contact with each other and easily form a path which becomes a heat path. Therefore, when a fibrous shaped thermally conductive filler is filled compared to the case of filling a spherical shaped thermally conductive filler, it is possible to obtain a high thermal conductivity, as described in Japanese Unexamined Patent Application Publication No H10-139893.
However, in order to obtain a high thermal conductivity, in the case where the amount of the thermally conductive filler which is filled is increased, there is a problem in that a resin composition rapidly becomes hard and the characteristics of the composite material significantly deteriorate. In order to solve such a problem, Japanese Unexamined Patent Application Publication No H3-200397 describes using two thermally conductive fillers in combination. That is, a plate-shaped filler is distributed in a layer shape in the longitudinal direction of a sheet and in multi-layers in the thickness direction forming a filler contained resin sheet with the particle shaped filler distributed between layers is described in the Japanese Unexamined Patent Application Publication No H3-200397.
However, with the miniaturization of electronic components in recent years and the progress in use of plastic parts, particularly in consumer electronic devices, a composite material having higher heat transfer properties has become necessary. Therefore, a carbon fibrous filler having excellent heat conductivity, for example carbon nanotubes or carbon fibers are attracting attention as a filler and especially a heat conductive filler.
As a heat transfer composite material which uses these carbon fibers as a thermally conductive filler, for example, a heat transfer material in which carbon fibers coated with a film having electrical insulating properties are uniformly dispersed in a synthetic resin having compatibility with the coating is described in Japanese Unexamined Patent Application Publication No H5-235217.
In the conventional technology as described above, when carbon fibers are added at a high filling amount, the carbon fibers with a high density begin to settle during the manufacturing process, and there is a problem whereby in the heat transfer composite material, non-uniformity of the distribution of the carbon fibers is generated between the front and rear surfaces. In such cases, there is a problem whereby heat resistance between heat generating components and heat dissipation components increases and cooling can no longer be performed efficiently. In addition, the creation of a composite material having a high thermal conductivity has been desired for sufficient cooling in small electronic devices and LED's.